Product Wrapper: response consistency / create & update methods

[drewwestphal]

Product XML response nodes were being inconsistently cast to their corresponding PHP types. The product create and product update methods took different parameters: create took a files array, update took a urls array.

This commit makes the following changes to address:

• Feed all product XML responses through a single static method to populate products (I believe this would be a fruitful path for the rest of the API Wrapper as well)
• Patch the create and update methods to take a $files and a $urls parameter (or no parameter), to ensure consistency and allow setting URLs with a single post call (my use case)